The invention relates to a matrix mold useful for the production of stackable containers, such as bowls and the like by molding of plastic foils. Preferably, these containers, carry on the wall one or more notches situated in the vicinity of the bottom or remote from it.
U.S. Pat. No. 3,172,159 teaches a matrix mold which interacts with a complementary matrix (male die). This matrix form contains at least one cup as boundary of a substantial truncated conical cavity widening upward and an ejector head which is well borne on the bottom of the cup.
The cup is provided with an annular stop springing back into the wall and with a counter-running conical ring between the edge of the stop and of the step, which is remote from the bottom of the mold cavity.
The counter-running conical ring of the cup prevents the ejection of the container, as it radially compresses the truncated conical ring of the container, and often impairs its appearance by furrows, cracks and dents which sometimes bring about a breakage.
The materials used in this mold, therefore, must be highly elastic and thereby also expensive, while the breakable and rigid materials, such as, for example, transparent polystyrene, are unsuited for use with such a mold.
Moreover, this mold, because of the limited radial dimensions of the stop yields containers which can be nested in one another with a variable stacking spacing. This results in containers which tend to stick or jam together, so that stacks of such containers cannot be easily used with automatic units.
If the counter-running conical ring of the container is locked in the vicinity of the bottom (see e.g. German Pat. No. 1,971,968 and Great Britain Pat. No. 865,025, FIGS. 4 and 5), and, namely, also if it consists of segments between there are present spacings along the circumference (see e.g. Great Britain Pat. No. 1,015,351), in agreement with the U.S. patent mentioned, then, the diameter of the ejector head has to be smaller than the smallest diameter of the mold cavity. For which reason in these cases arching and deformation of the container bottom during ejection from the mold will likely occur.